The present invention relates to magnetotelluric surveys and, more particularly, to improved methods for processing magnetotelluric signals.
There are many different methods for locating hydrocarbon deposits, ore bodies, water, and other natural resources in the earth's crust. Drilling test holes in an area of interest is the most direct method. Samples from various depths may be obtained and analyzed for evidence of commercially exploitable deposits. Test drilling, however, is extremely expensive and time consuming. Thus, it is rarely a practical option for exploring unknown and unproven areas.
Seismic surveys are one of the most important techniques for discovering the presence of hydrocarbon deposits. A seismic survey is conducted by deploying an array of energy sources, such as dynamite charges, and an array of sensors in an area of interest. The sources are discharged in a predetermined sequence, sending seismic energy waves into the earth. The reflections from those energy waves or “signals” travel through the earth, reflecting or “echoing” off various subsurface geological formations. Inferences about the depth of those formations may be made based on the time it takes the reflection signals to reach the array of sensors.
If the data is properly processed and interpreted, a seismic survey can give geologists an accurate picture of subsurface geological features. Seismic surveys, however, only identify geological formations capable of holding hydrocarbon deposits. They do not reveal whether hydrocarbons are actually present in a formation, nor do they provide information from which one may infer the presence of metallic ores. Moreover, the time and expense involved in conducting a seismic survey, while considerably less than that of test drilling, is nevertheless substantial.
Geological surveys also have been based on the detection and interpretation of magnetotelluric signals. Magnetotelluric radiation emanates from the earth and may be caused by current flow in the upper layers of the earth's crust. The current flow in turn creates electromagnetic fields adjacent to, but above the earth's surface that are directly related to the resistivity of the earth through which the induced current is flowing. That resistivity in turn may be used to infer the presence or absence of valuable deposits. For example, areas of increased resistivity may indicate the presence of hydrocarbons since hydrocarbons are poor conductors. Areas of lower resistance may indicate the presence of valuable metal ores which are relatively good conductors.
Magnetotelluric surveys also are much less expensive than seismic surveys. There is no need to install an array of sources and receivers across what may be a very substantial area to be surveyed as in seismic surveying. Instead, magnetotelluric detection equipment and recorders may be carried across the survey area by truck, all-terrain vehicle, helicopter, or other mode of transportation suitable for the survey area.
Despite the considerable theoretical and practical advantages of magnetotelluric surveying, however, its promise has not been fully realized, so much so that such surveys are often met with the skepticism normally reserved for water witching, divining and the like. That perception has been created in large part because many conventional magnetotelluric methods are based on converting magnetotelluric signals into audio signals that are then aurally interpreted by an operator. Obviously, the reliability and consistency of such methods, to the extent they exist at all, is dependent on the ability of the operator to hear differences in the signals and to properly interpret them.
Other methods have focused on detection and interpretation of the DC component of magnetotelluric fields. For example, U.S. Pat. No. 4,945,310 to J. Jackson et al. discloses methods based on measuring the potential created across a pair of spaced electrodes. The AC component of the potential is filtered out, leaving a DC potential the magnitude of which is functionally related to the subsurface lithology at the detection site. U.S. Pat. No. 4,473,800 to B. Warner and U.S. Pat. No. 5,770,945 to S. Constable also disclose methods of detecting and analyzing the DC component of magnetotelluric signals using dipole antennas that detect both the magnetic and electrical components of magnetotelluric fields.
The applicability of such methods, however, is severely limited. The presence and strength of DC signals is dependent on the time of day and weather conditions. For example, they are extremely difficult to detect reliably during overcast periods and during rainstorms, and they are almost undetectable at night. More importantly, however, the DC component of magnetotelluric fields has no correlation to depth. Thus, while the DC component may be analyzed to make inferences about the overall resistivity of the earth below a survey location, it is impossible to deduce the resistivity of the earth at specific depths, or to detect differences in resistivity at different depths.
Other methods focus on detecting and interpreting the extremely low frequency AC component of magnetotelluric signals. Such signals typically are below about 3 kHz. There is a direct relationship between a given magnetotelluric frequency and subsurface depth. Thus, the resistivity of the earth at a particular depth is related to the amplitude of the signal at a corresponding frequency. For example, the resistance of a shallow subsurface formation can be measured by detecting and analyzing higher frequency magnetotelluric signals. The resistance of deeper formations can be measured by analyzing lower frequencies.
For example, U.S. Pat. No. 5,777,478 to J. Jackson discloses methods of detecting and analyzing the AC component of magnetotelluric signals. Those methods entail modulating and then demodulating a magnetotelluric signal with a sweep oscillator. The sweep oscillator beats the received signal with a generated signal to generate tuned signals at various frequencies. The tuned signals then are converted to pulses by reference to a threshold value. That is, whenever the tuned signal exceeds a predetermined threshold value a pulse is generated. The number of pulses over a given time period, what is referred to as the “pulse density”, is said to provide a measure of conductivity relative to other depths and locations in the survey area.
Magnetotelluric signals, however, are extremely weak and typically are very noisy. Prior art methods have not provided effective methods for improving the quality of magnetotelluric signals, i.e., their signal to noise ratio. Jackson '478, for example, teaches the use of a relatively large bandwidth low-pass filter. Such filters pass a relatively large spectrum and quantity of noise along with the signal to be analyzed.
Jackson '478 also bases its analysis of magnetotelluric signals on “snap shots” of the data. That is, it suggests that the tuned signals generated at each location should not be maintained for long periods of time so as to avoid any fluctuations in the overall strength of the received signal that might introduce unnecessary error in the survey. At the same time, however, the accuracy of the overall survey depends on an unstated, though faulty assumption that the received signals are relatively constant, since data is being collected and analyzed from various locations in the survey at different times. Moreover, by relying on “snap shots” of fluctuating signals, the results of such methods are difficult to replicate from survey to survey.
Thus, to date there has been little success in systematically analyzing magnetotelluric signals despite the availability of quiet detection and recording equipment and efficient and powerful digital computers. Such equipment makes it possible to easily acquire and process large amounts of data. It is believed, therefore, that the lack of success in large part derives from the inability of the prior art to recognize the essentially chaotic nature of magnetotelluric signals and to construct effective models for isolating and identifying meaningful data in magnetotelluric signals. Whatever the reason, the fact is that conventional methods of processing magnetotelluric data have not been sufficiently effective or efficient for magnetotelluric surveying to gain commercial acceptance or widespread use.
An object of this invention, therefore, is to provide improved methods for conducting geological surveys and, more particularly, methods that are relatively inexpensive as compared to test drilling and seismic surveys and yet still accurately identify the presence of hydrocarbons, ore bodies, water, and other natural resources in the earth.
A more specific object of the subject invention is to provide improved methods for processing magnetotelluric signals that may be processed by conventional digital computers and that do not rely on an operator to distinguish differences in a magnetotelluric signal.
It also is an object to provide such methods that more effectively remove unwanted noise and identify and analyze meaningful components of magnetotelluric signals.
Another object of this invention is to provide such methods that more accurately and reliably reflect the relative resistivity of subsurface geology across a survey area, and especially, such methods that do so despite variations in the strength of magnetotelluric signals as the signals are detected and recorded during the course of a survey.
Yet another object is to provide such methods wherein all of the above-mentioned advantages are realized.
Those and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.